This invention relates to a glass substrate which is mainly used for an electron device and which is capable of patterning with high accuracy, a photomask blank and a photomask using the glass substrate.
The known photolithography method is generally used to form a fine pattern in a step for fabricating a semiconductor integrated circuit and a photomask.
For instance, the pattern is transferred on a transparent substrate by the use of the photomask. Herein, the transparent substrate is finished to a mirror surface by accurately polishing it. Further, the photomask is made by forming the pattern thereon by the use of a light shielding film, such as, a chromium film.
Recently, strict condition has been required for fine defects (contaminant on a surface, flaw, and stain) with respect to the mirror-finished transparent substrate which is polished with high accuracy and high density.
An inspecting method for removing the fine defects from the transparent substrate has been suggested in Japanese Unexamined Patent Publication No. Sho 58-162038 (thereinafter, referred to as a conventional reference).
In this conventional reference, a light beam is focused into a fine region on a pattern surface. In this condition, a surface state of the substrate is inspected by comparing a reflected output with a transmitted output from the pattern surface.
However, in the conventional reference, only when the light beam is irradiated to a constant direction, the inspection is carried out by comparing the reflected output with the transmitted output.
In consequence, it is difficult to accurately detect a fine flaw having a specific polarity on the surface of the glass substrate even when the latest inspection apparatus is used. Further, the defect, such as the stain, inside the glass substrate can not be detected at all.
In this case, the flaw having the specific polarity is formed via a trace through which an undesirable contaminant passes by mixing the contaminant during a polishing step of the glass substrate. Alternatively, the flaw may be formed when the glass substrate is inserted into a supporting case during moving the glass substrate.
Further, the flaw may be also formed when the glass substrate is handled after the polishing step. In this event, it is difficult to detect the flaw by the normal inspecting method or apparatus disclosed in the conventional reference.
Moreover, it is assumed that the size of the flaw having the specific polarity on the surface of the glass substrate is represented by a length of a major axis direction and a length of a minor axis direction perpendicular to the major axis direction.
In this condition, the flaw having the length of the minor axis direction of 1 xcexcm on a surface in which the flaw is cut by a cross sectional plane perpendicular to the surface can not be accurately detected in the conventional reference.
Herein, it is to be noted that when a size of a concave portion on a principal surface is hereinafter described, the size is represented by (the length of the major axis direction and the minor axis direction perpendicular to the major axis direction), and each length of the major axis direction and the minor axis direction indicates the length on the principal surface when the concave portion is cut by a cross sectional plane perpendicular to the principal surface.
In this case, it is assumed that the surface state of the glass substrate is inspected by the conventional reference, and the photomask and the phase shift mask are manufactured by the use of the inspected glass substrate.
Under this circumstance, even when the glass substrate, in which no defect is detected in the inspection, is used, the desired pattern can not be obtained when the pattern is transferred to a substance to be transferred because of pattern breakage (or clear extension). This problem takes place by the following reason.
Namely, when the defect, such as, the flaw on the surface of the substrate has dependency for a moving direction of an inspection light beam, the defect has polarity of a constant direction. Consequently, the defect may be not detected in accordance with an incident direction of the inspection light beam.
Further, when the defect, such as, the stain exists inside the substrate, the reflected output is not almost detected in the conventional reference. From these reasons, it is accidentally judged that the glass substrate has no defect in the conventional reference.
It is therefore an object of this invention to provide a glass substrate for an electron device, a photomask blank and a photomask which are capable of performing patterning and projection lithography with high accuracy.
In a glass substrate for an electron device which is selected based upon a predetermined reference set value using a method for detecting a defect for the glass substrate in accordance with optical change of an inspecting light beam, the defect for the glass substrate has no dependency for a moving direction of the inspecting light beam.
Conventionally, the inspecting light beam is introduced into a constant direction. The glass substrate is inspected by comparing the reflected output with the transmitted output. In this condition, the defect (for example, the flaw on the surface of the substrate) which has dependency in the moving direction of the inspecting light beam can not be accurately detected and removed.
Namely, the reflected output is not detected at all in many cases because only a light beam of a constant direction passes for the flaw.
In contrast, in the case of an inspecting method for inspecting non-uniformity of a translucent substance disclosed in Japanese Patent Application No. Hei 9-192763 (thereinafter, will be referred to as a related reference and will be described in more detail), the light beam is irradiated from an inner portion of the substrate toward the surface thereof in all directions for the flaw.
Thereby, even when the defect has dependency in the moving direction of the inspecting light, the defect can accurately be detected and removed.
In consequence, the pattern defect does not take place in the etching process during the patterning. As a result, the patterning can be carried out with high accuracy. Further, the exposure can be suitably performed by the use of the photomask and the phase shift mask using the glass substrate.
Herein, the optical change means that optical characteristic, such as, optical quantity, the moving direction of the light beam is varied.
Further, the reference set value means optical information (image information, light quantity, brightness, and strength distribution and the like) which is obtained in accordance with a permissible defect (having non-uniformity) for the glass substrate, and means a threshold value which is flexibly set by a user.
The inspecting light beam is introduced into the glass substrate so that the light beam transmits by repeating total reflection on the surface of the glass substrate when a light path of the glass substrate is optically uniform.
Namely, the light beam which satisfies all reflected condition is introduced into the glass substrate. Thereby, when the light path is optically uniform inside the glass substrate, the light beam covers the entire surface of the substrate in all directions.
As a result, even when the concave portion (the flaw) having the specific polarity exists on the surface of the glass substrate or the defect, such as, the stain exists inside of the glass substrate, the defect can be accurately detected with high accuracy and high speed.
The inspecting light beam is a laser light beam. Further, the surface of the glass substrate includes at least a pair of principal surfaces parallel to each other, at least a pair of side surfaces perpendicular to the principle surfaces, and a chamfered surface interposed between the principal surface and the side surface. With such a structure, the introduced laser light beam transmits inside the glass substrate so that the total reflection is carried out through the principal surfaces and the side surfaces and the laser light beam repeatedly makes a round trip at least between a pair of the side surfaces when the light path of the glass substrate is optically uniform. Thereby, the laser light beam transmits across a region which is to be inspected and is surrounded by the principal surfaces, the side surfaces and the chamfered surface.
In this condition, the introduced laser light beam repeats the total reflection through the principal surfaces, the side surfaces. Consequently, it is easy that the laser light beam is substantially sealed in the glass substrate. As a result, the inspection is practically carried out in a wide range region at the same time so that the inspection can be preferably performed at high speed. Namely, incident angles of the laser light beams are identical to each other at the principal surface on which the total reflection is repeated for the introduced laser light beam. Further, the incident angles of the laser light beams entered to the side surfaces are also identical to each other. under this circumstance, the laser light beam transmits with a constant relationship. In this relationship, when the incident angle of the laser light beam at the principal surface is selected to xcex8, the incident angle of the laser light beam entered to the side surface becomes 90xc2x0-xcex8. In this condition, the incident angle for the principal surface, through which the introduced laser light beam initially passes, is set so as to be larger than the critical angle. Further, the incident angle for the side surface is also selected so as to be larger than the critical angle. Thereby, the laser light beam is substantially sealed in the glass substrate.
The light beam is introduced from the chamfered surface, and the laser light beam is outputted from only the chamfered surface when the light path of said glass substrate is optically uniform, as a specific introducing method of the laser light beam in.
In a glass substrate for an electron device which has a concave portion on a principal surface of said glass substrate, the concave portion is represent by a first length of a major axis direction and a second length of a minor axis direction perpendicular to the major axis direction in size.
In this event, the second length on the principal surface specified when the concave portion is cut by a cross sectional plane perpendicular to the principal surface is 1 xcexcm or less. Herein, the principal surface is positioned at a side in which a pattern is formed.
Even when a small defect of 1 xcexcm or less exists in the minor axis direction as the size of the concave portion (the surface defect, such as, the flaw) of the glass substrate, the pattern defect does not occurs in the etching process during the patterning.
Consequently, the patterning can be carried out with high accuracy. Further, the exposure can be accurately performed by the use of the photomask and the phase shift mask using the glass substrate.
In this event, the concave portion having the size of about 0.05 xcexcm in the minor axis direction can be accurately detected by the use of the inspecting method and the inspecting apparatus disclosed in the related reference.
The second length is 0.5 xcexcm or less. When the second length is 0.5 xcexcm or less, generation of the pattern defect is suppressed in the etching process during the patterning. Consequently, it is possible to pattern with high accuracy, and the reliability is largely improved.
The second length falls within the range between 0.05 and 0.25 xcexcm. When the second length is less than 0.25 xcexcm, although the quality may be improved, the manufacturing yield is degraded, and further, the manufacturing cost is increased.
In the glass substrate, a non-uniform portion, in which optical characteristic for a transmitted light beam is non-uniform, does not substantially exist inside the glass substrate.
This means that the non-uniform portion, such as stain or bubble, does not exist in the glass substrate. Further, this means that even when the exposure is carried out by the use of the photomask and the phase shift mask using the glass substrate, the optical strength distribution of the exposure light beam on the subject to be transferred falls within the permissible range. Namely, the non-uniform portion has excessively small non-uniformity in the above permissible range and it is negligible as the defect.
When the non-uniformity is not detected by the use of the inspecting method and the inspecting apparatus disclosed in the related reference, the glass substrate sufficiently satisfies the above condition.
As mentioned before, according to this invention, when the size of the concave portion (the surface defect, such as the flaw) on the principal surface of the glass substrate is represent by (the length of the major axis direction and the minor axis direction perpendicular to the major axis direction), the length of the minor axis direction on the principal surface specified when the concave portion is cut by a cross sectional plane perpendicular to the principal surface is equal to 1 xcexcm or less.
Consequently, the pattern defect does not occur in the etching process during the patterning. As a result, the patterning can be carried out with high accuracy.
Further, non-uniform portion of the optical characteristic for the transmitted light beam does not substantially exist inside the glass substrate. In consequence, the exposure can be properly performed by the use of the photomask and the phase shift mask.
Moreover, the defect, which exists for the glass substrate, has no dependency for the moving direction of the inspecting light beam. Consequently, the pattern defect does not bring about in the etching process during the patterning. As a result, the patterning can be carried out with high accuracy. In addition, the exposure can be properly performed by the use of the photomask and the phase shift mask.